I have 92 400E with 165K Mi. My latest problem is when I first start the car the oil pressure reads 3 BAR and stays there at idle and driving but after the car warms up to 90C the presure drops to 0 even driving down the hwy. I search this site for this problem but I did not find the same situation. Any ideas what it may be? I found two leads; oil tubes and oil relief/check valve. But I did not see the same simptoms described in the threads.
Where should I start probing?

I'd try and check the oil pressure sender. Your engine would probably be damaged by now if the oil pressure was actually going to zero. Do you get an idiot light when the pressure drops, or does it just indicate low pressure on the gauge?

Troy
1995 E420

If your oil pressure is really zero on the highway the engine would seize up in a matter of seconds.

The likely problem is the electic oil pressure sending unit or the gage.

The pressure should be checked with a bourdon tube test gage to be sure it is in the proper range. If it's okay, the most likely bad actor is the oil pressure sending unit on the engine.

Duke

The only idiot light is the oil level light and since the oil level is fine the light does not come on. I suspected as much if it was not getting any oil I would have felt it by now since I have put on ~8 K mi. since the start of this problem. First I thought the oil has thinned out too much so I added oil additive (viscosity improver) but it made no difference. I would like to test the oil pressure with a mechanical pressure guage that I have but I do not know where to attach it?

I checked the oil pressure with a mechanical gage and it registered 22 PSI (~1.5 Bars) at idle when engine hot while the pressure gage in the car reads 0. This indicated to me the oil pressure sender needs to be replaced.

I did finally change the oil pressure sensor and the car gage pressure reading is back to normal. I just wanted to complete the thread.

where is the oil pressure sensor in a 95 E420? my vehicle is also reading 0.

It's just below the oil filter housing. Buy the new one or find a picture of one on the internet (like under "buy parts" above) and you can't miss it.